What is the religious ruling on the deceased's family providing an expensive banquet (called "ta'leef") and other meals for attendees for days after the death, and is there anything that necessitates providing them as charity for the deceased? And what is the ruling on mourning for forty days for the deceased? And what is the ruling on the wadheema?

The situation mentioned in the question falls under wailing (niyaha), which is forbidden in Islamic law. The Sunnah in this matter is for neighbors or relatives to prepare food for the family of the deceased, as they are preoccupied with their bereavement. Muslims must follow the Sunnah of the Prophet, peace and blessings be upon him, and abandon customs that contradict the Sunnah and burden those afflicted, which may lead to debt, consuming the wealth of orphans, or extravagance. The family of the deceased is not obligated by Islamic law to give charity on behalf of the deceased.